Python Function to Discover Triangle Slope
Okay, here's a start:

import bpy
from mathutils import Vector

a = Vector((..., ..., ...))
b = Vector((..., ..., ...))
c = Vector((..., ..., ...))

ab = b - a
ac = c - a

normal = ab.cross(ac)
normal.normalize()

# assuming Z is up
slope_dir = normal.xy


# Heres 3 methods you could use to measure slope amount...

# measure xy length (0-1) where 1 is flat.
slope_amount = slope_dir.length
# so you can tell if the slope points up or down...
slope_amount = slope_dir.length * (1.0 if normal.z &gt; 0.0 else -1.0)

# use normal z axis, 1.0 will be up, -1.0 down.
slope_amount = normal.z

# measure slope angle (0 - pi) where 0 is flat.
slope_amount = Vector((0.0, 0.0, 1.0)).angle(normal)